1. Field of the Invention
The present invention relates to headsets, and more particularly but not exclusively to headsets utilizing active noise cancellation.
2. Description of the Related Art
Conventionally, passive headsets comprise a pair of earpieces coupled by a resilient headband. An annular foam pad attached to each earpiece forms a cushion between the shell of the earpiece and the user""s head. The resilient headband presses the earpieces against the user""s head. Ambient sound is attenuated before it reaches the wearer""s ear by occlusion of sound by the earpieces and absorption of transmitted sound by materials within the earpieces. The degree of attenuation achieved depends upon the nature of the ambient noise and the qualities and characteristics of the individual headset.
In various applications, however, passive attenuation is insufficient. Some environments are simply too noisy for comfort or even safety with only passive earpieces. In other environments, the elimination of extraneous noise is paramount, and satisfactory results cannot be achieved using passive means. Although the amplitude of the extraneous noise may be significantly diminished, it is almost impossible to completely isolate the wearer from extraneous noise using passive means. In addition, passive earpieces attenuate all sound, regardless of whether the wearer needs or wants to hear particular sounds.
Active noise cancellation systems eliminate unwanted sound using destructive interference. Cancellation is achieved by propagating anti-noise, identical to the unwanted soundwaves but inverted, which interacts with the unwanted waveform and results in cancellation. A feedback active cancellation headset typically includes a sound generator in each earpiece for producing anti-noise, and a residual microphone, also located in each earpiece, to provide feedback signals to a controller which generates the proper anti-noise signals. Each microphone detects the unwanted noise within each earpiece and provides corresponding signals to the controller. The controller supplies anti-noise signals to the sound generator corresponding to the noise detected in the earpieces, but inverted, with respect to the unwanted waveform. When the anti-noise interacts with the noise within each earpiece, destructive interference between the noise and the anti-noise cancels the unwanted sound.
A problem associated with feedback cancellation systems is that they are prone to instability. Feedback systems tend to become unstable, for example, if the bandwidth of the system is too broad or the gain of the system is too high. When instability occurs, the system usually emits a loud noise that is generally unpleasant and occasionally dangerous. Consequently, the maximum range and effectiveness of feedback systems are limited by parameters designed to keep the feedback system stable.
To effect maximum cancellation, the waveform of the interacting anti-noise should exactly match the unwanted waveform, but should be inverted. The acoustic properties of each earpiece, however, affect the characteristics of the anti-noise waveform. The effect of the acoustic properties may be corrected by processing the residual signal according to a transfer function characteristic of the acoustic properties of the system to compensate for the effects. However, these acoustic properties of the headset are not constant under all conditions, and may vary with the force applied to the earpiece onto the user""s head. When high pressure is applied to the earpiece, or when the headset is removed from the user""s head, the variation of the earpiece""s acoustic properties, particularly the volume and acoustic resistance, may cause instability in the feedback loop. This instability in turn, causes the control loop to generate unstable oscillations, producing unpleasant and potentially even harmful noise.
Moreover, when a headset is designed for listening to music or information broadcast over a vehicle entertainment system or the like, the audible sound is affected to some extent by conditions at the earpieces. Thus, if the volume contained between the user""s ear and the earpiece is changed, for example because the user rests the side of the head against a pillow or the like, sound distortion can occur due to a change in frequency response, especially high frequency response.
The headset in accordance with the invention limits the changes in reproduction of feedback sound which can take place in an earpiece due to compression of the cushion via which the earpiece is pressed against the user""s ear.
Thus, according to one aspect of the invention, there is provided a headset comprising an earpiece adapted to be held against the ear of the user, said earpiece comprising:
an earpiece shell;
a baffle plate incorporated in the earpiece shell;
a sound generator carried by the baffle plate; and
a cushion carried by the earpiece shell and via which the earpiece is adapted to be held against the ear;
wherein the baffle plate carries means for limiting compression of the cushion against the ear.
When the headset is receiving music or other sound, for example from a vehicle entertainment system, compression of the earpiece cushion is restricted to minimise distortion if the cushion is compressed, for example because the user presses by hand on the earpiece or leans his or her head against a pillow.
The invention is applicable to earpieces having a cushion in the form of a cushioning ring surrounding the sound generator or drive unit, and earpieces having a cushion overlay. In the former case at least, an apertured dome may be provided above the sound generator, i.e. upstanding from the central region of the baffle plate, to restrict compression of the cushion. In the latter case, the baffle plate may carry upstanding, flat headed pins distributed over its surface which in use faces towards the ear.
The cushion compression limiting means preferably restricts compression to less than 70 per cent, preferably less than 50 per cent, for example, of the maximum cushion compressibility.
The invention is even more useful, however, in a headset providing for active noise cancellation of unwanted environmental noise.
Thus, according to another aspect of the invention, there is provided a headset for active noise cancellation of unwanted environmental noise, comprising an earpiece adapted to held against the ear, said earpiece comprising:
an earpiece shell;
a drive unit incorporated in the shell;
a microphone incorporated in the shell to detect unwanted noise and feed via an inverting amplifier a phase inverted, unwanted noise cancellation signal to the drive unit;
a cushion carried by the earpiece shell; and
means for restricting compression of the cushion against the ear to the extent necessary substantially to prevent acoustic gain increasing beyond a threshold which gives rise to the generation of sound oscillations at the earpiece.
If the user acts in any manner which would normally give rise to such compression of the cushion that highly unpleasant oscillations,especially high frequency oscillations, would be heard in the ear, the cushion compression restricting means, for example an apertured dome as aforesaid or any other convenient form of upstand, restricts cushion compression to the extent necessary substantially to prevent these oscillations from developing.
The cushion restricting means, if located centrally, also presses on the ear flap (tragus) when the cushion is compressed to the maximum possible extent, to close the ear canal (crus helias) against any residual oscillatory noises which may occur.
Moreover, the headset may include cushion deforming means for causing the cushion, when held against the ear, to close any gap which would otherwise exist between the side of the head and the front of the ear forwardly of and above the ear canal.
This cushion deforming means may comprise an off-centre protrusion from the baffle plate, aligned with the gap which exists between the front of the earpiece and the side of the head when the earpiece is properly fitted by the headband, this protrusion, e.g. a generally radial ridge, being of lesser height than the upstand or upstands which restrict cushion compression.
Also, when the earpiece is properly fitted, the microphone for detecting unwanted environmental noise is preferably positioned to align with the intertragel incisure leading to the ear canal.
The invention also provides a complete headset system comprising:
an earpiece adapted by a headband to be held against the ear, said earpiece comprising:
an earpiece cup;
a drive unit carried by the cup;
a microphone carried by the cup;
an ear cushion carried by the cup; and
means carried by the cup for limiting compression of the cushion against the ear;
an electronics unit, said electronics unit completing a feedback loop which includes an inverting amplifier means adapted to generate an amplified, phase inverted signal from an unwanted noise signal received from the microphone and to feed said phase inverted signal to the drive unit, the acoustic gain of the loop being increased when the earpiece cushion is compressed against the ear beyond a predetermined extent;
wherein the compression limiting means carried by the earpiece is adapted to restrict compression of the cushion not substantially to exceed said predetermined extent.
The electronics unit is preferably battery powered and mounted remotely from the headset, although this unit could be mounted on the back of the earpiece shell, optionally with the power source remotely located.
The headset system is applicable to portable personal music systems worn by an individual user, but more particularly finds use in vehicles having entertainment and/or passenger announcement systems.
Thus, still further, the invention provides, for use in a vehicle having an entertainment system and at least one passenger seat, a headset system comprising:
earpieces interconnected by a headband, whereby the earpieces are adapted to be held against the ears of a passenger in the seat, each earpiece carrying:
a sound generator for broadcasting sound transmitted as an electrical sound signal by the vehicle entertainment system;
a microphone;
an earpiece cushion; and
means limiting compression of the cushion against the ear;
the system also including an electronics unit mounted to the passenger seat, said electronics unit being adapted for connection to the earpieces and including:
an input circuit for receiving the entertainment sound signal;
an inverting amplifier for receiving a signal representing unwanted noise from the microphone and for generating an amplified phase-inverted signal therefrom;
a summing circuit for summing the said entertainment signal and the phase-inverted signal; and
an output circuit for leading the summed signal to the sound generators in the earpieces, thereby effectively to cancel unwanted environmental noise present at the earpieces;
wherein the cushion compression limiting means serves to limit compression of the cushion to an extent beyond which acoustic feedback from the microphones to the sound generator via the electronics unit tends to generate audible oscillations in the earpieces.
It is also conceivable that the invention may be applied to the adaptation of existing headsets which incorporate a flat baffle plate which provides no restriction for compression of the earpiece cushion.
Thus, in accordance with another aspect of the invention, there is provided, for an earpiece adapted to be held against the ear and comprising an earpiece shell, a generally flat baffle plate incorporated in the shell, a sound generator carried by the baffle plate and an earpiece cushion carried by the baffle plate; an earpiece attachment comprising an element having means of attachment to the baffle plate and an upstanding part which, when the attachment is secured to the baffle plate, serves to limit compression of the earpiece cushion.